SECONDARY LIMESTONE. 435 



imbedded organic remains. All these are mat- 

 ters of a highly interesting nature, but the most 

 general notice of them only is admissible in this 

 place. 



Some remarkable varieties of internal struc- 

 ture are found in the secondary limestones. 

 That which has been called the schistose, is im- 

 properly so named, as it is, more strictly, laminar. 

 Almost all the laminar limestones seem to owe 

 their fissility, either to the presence of certain 

 organic remains, or to the interposition of shale 

 or clay, however small in quantity. Although 

 these forms, therefore, may often be very remark- 

 able, they cannot be considered as examples of 

 an internal structure. Even the most thinly 

 laminar limestones, which are often so valuable 

 for economical purposes, appear to derive that 

 form from their stratification ; and the laminae 

 ought to i be considered as thin strata separated 

 from each other without an intervening substance, 

 as the larger frequently are. I am not aware 

 that any example has occurred in the calcareous 

 rocks, of a fissile tendency not conforming to the 

 planes of stratification. Should such occur, as 



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